1. Field of the Invention
The present invention relates to a method of manufacturing a solid state image sensing device, which forms a light shielding film on a semiconductor substrate provided with a photoelectric converting area and a transfer channel for receiving charges from the photoelectric converting area and transferring the charges. More particularly, this invention relates to a method of manufacturing a solid state image sensing device with an improved method of forming a light shielding film.
2. Description of the Related Art
In a solid state image sensing device like a CCD (Charge Coupled Device) image sensor, a plurality of photoelectric converting elements are arranged in a line or two-dimensionally. When an image is projected on those photoelectric converting elements, electric signals proportional to the brightnesses of the image are output from the respective photoelectric converting elements as a result of sensing the image.
FIG. 1 is a plan view showing a unit pixel of an ordinary conventional CCD image sensor, and FIG. 2 is a cross-sectional view of the unit pixel. An N type diffusion layer 2 (photoelectric converting area) is formed on the surface of a P type silicon substrate 1. This P type silicon substrate 1 and the N type diffusion layer 2 constitute a PN photodiode or a photoelectric converting element. Further, a P.sup.30 type diffusion layer (device isolating area 3) and an N type buried channel 4 are formed on the substrate surface. First transfer electrodes 7-1 and second transfer electrodes 7-2 are alternately arranged, partially overlapping one another, on the N type buried channel 4 via a gate insulating film 6. The first and second transfer electrodes 7-1 and 7-2 are formed of a polysilicon film. An insulating film 8 is formed about 200 nm thick by thermally oxidizing the first transfer electrode 7-1. This insulating film 8 insulates the first and second transfer electrodes 7-1 and 7-2 from each other. The N type buried channel 4, the first transfer electrode 7-1 and the second transfer electrode 7-2 constitute a vertical CCD shift register. The second transfer electrode 7-2 also serves as a shift electrode for reading charges accumulated in the N type diffusion layer 2 and transferring the charges via a transfer channel 5 to the N type buried channel 4. A light shielding film 10 made of aluminum or the like is provided on the first and second transfer electrodes 7-1 and 7-2 via an interlayer insulating film 9. This light shielding film 10 has an opening 11 formed only at the portion above the photoelectric converting area (N type diffusion layer 2), exposing the interlayer insulating film 9 there.
The operation of the conventional solid state image sensing device in this unit pixel section will be described below. A signal charge produced by the photoelectric conversion of incident light 12 is accumulated in the N type diffusion layer 2, and is read out to the N type buried channel 4 via the transfer channel 5 by the shifting operation of the shift electrode. Subsequently, the accumulated signal charges are transferred in the N type buried channel 4 in response to a transfer pulse applied to the first and second transfer electrodes 7-1 and 7-2.
The N type buried channel 4 and the P type silicon substrate 1 have a PN junction, so that when light enters this portion, photoelectric conversion occurs, thus generating unnecessary charges. If the unnecessary charges are mixed with the signal charges read from the N type diffusion layer 2, the accurate image signal cannot be obtained. To prevent this phenomenon, the light shielding film 10 made of aluminum or the like is provided above the first and second transfer electrodes 7-1 and 7-2 via the interlayer insulating film 9. This light shielding film 10 permits light 12 to be incident only on the photoelectric converting area (N type diffusion layer 2) and prevents the light incident to the other areas.
As CCD image sensors are designed to have a greater number of pixels to improve the resolution, the area per pixel becomes smaller and the area of the opening in the light shielding film for receiving the incident light becomes smaller and smaller. To minimize the reduction in sensitivity due to the reduced pixel area, the effective use of light is desired. When an aluminum film is used as the light shielding film 10, the aluminum film should have a thickness of 0.2 .mu.m from the viewpoint of only the light transmittivity. But, pin holes are easily formed in the aluminum film. To prevent the light transmission through the pin holes, the aluminum film should have a thickness of about 0.5 .mu.m.
There is a step formed at the edge portion of the photoelectric converting area (N type diffusion layer 2) by the first and second transfer electrodes 7-1 and 7-2 etc. Supposing that the thickness of the gate insulating film 6 is 80 nm, the thicknesses of the first and second transfer electrodes 7-1 and 7-2 are 250 nm and the thickness of the insulating film 8 of a thermally oxidized polysilicon film, which insulates those transfer electrodes 7-1 and 7-2 from each other, is 200 nm, there is a step of about 0.5 .mu.m at the A--A line portion in FIG. 1. In further consideration of the step coverage of the interlayer insulating film 9 (e.g., a polysilicon oxide film of 300 to 600 nm thick formed by a CVD method), there is a step of around 0.8 .mu.m. Accordingly, the step coverage of the aluminum film is impaired and the flat portion of the aluminum film should have a thickness of 0.8 .mu.m or above. As the light shielding film 10 becomes thicker, light 12a obliquely incident in the vicinity of the opening 11 is reflected by the edge portion of the light shielding film 10 as shown in FIG. 2, thus interfering with the effective usage of light. If the light shielding film 10 is so formed as to completely cover the side wall portion of the transfer electrode in order to obtain a good smear characteristic, the aspect ratio becomes higher because of the necessity of fine processing to provide an opening of 1 .mu.m or below due to the increasing multipixel structure. This makes it very difficult to form such an opening with a high precision. Further, since the diameter of the opening is determined by the thickness of the light shielding film which covers the side wall portion of the transfer electrode, it is difficult to form the opening beyond a certain diameter due to this restriction.
As one way to solve this problem, a metal film having a high melting point has been used conventionally because of its excellent step coverage and its resistance against the formation of pin holes. The film of a refractory metal has a reflectance which is about 50% of that of an aluminum film. When this metal film is used as a light shielding film, it prevents the reflection of light at the end faces and the back (which contacts the interlayer insulating film 9) of the light shielding film and reduces the smear. But, the light transmittivity of the metal film is greater than that of the aluminum film. If a tungsten film is used as the light shielding film, for example, it should have a thickness of 0.4 .mu.m or above in order to prevent the light transmission. Therefore, there is a limit to making the light shielding film thinner.
A known conventional method which improves the step coverage of an aluminum film is disclosed in Unexamined Japanese Patent Publication No. Sho 63-53949. According to this method, after an aluminum film is formed by sputtering, it is subjected to a heat treatment at a temperature of 575.degree. to 720.degree. C. so that the aluminum film is melted to reflow the aluminum film at the step portion.
FIG. 3 is a cross-sectional view showing the metal wiring of a semiconductor device described in the aforementioned Japanese publication. An SiO.sub.2 film 102 is formed on a silicon substrate 101. A contact hole 107 is formed in this SiO.sub.2 film 102, and an impurity diffusion layer 104 is formed on the surface of the substrate 101 in this contact hole 107. Then, an aluminum film 106 is formed 0.3 .mu.m thick on the surface of the SiO.sub.2 film 102 by sputtering. The resultant structure is then subjected to a heat treatment at 700.degree. C. FIG. 3 shows the resultant state. A polysilicon film 103 is buried in the SiO.sub.2 film 102 at a protruding step 108. The thickness of the aluminum film 106 above the protruding step 108 is about 0.1 .mu.m. Reference numeral "105" denotes a titanium nitride film having a thickness of about 0.2 .mu.m.
As apparent from the diagram, the aluminum film 106 becomes thinnest at a shoulder 109 of the protruding step 108. The aluminum film is almost completely buried in the contact hole 107 having a diameter of about 1 .mu.m. The thickness of the SiO.sub.2 film 102 in the contact hole 107 is about 1 .mu.m.
If this conventional method is used to form the light shielding film of a solid state image sensing device, it is apparent that the aluminum film becomes thin on the transfer electrode and at the shoulder portion (portion 9a in FIG. 2) of the step portion but becomes thicker on the photoelectric converting area. If pin holes hardly exist in the aluminum film after melting process, the aluminum film at the step should have a thickness of at least 0.2 .mu.m. One cannot specifically say how thick the aluminum film should be deposited by sputtering. Assuming that the aluminum film needs to be approximately twice as thick as 0.3 .mu.m, the thickness above the photoelectric converting area becomes 0.6 .mu.m or above, so that the effect of the melting process cannot be expected so much. An aluminum film may be deposited again by sputtering after the melting process. In this case, when an aluminum film of 0.3 .mu.m thick is deposited and melted first, the thickness of the shoulder portion 9a becomes about 0.1 .mu.m, requiring re-deposition of an aluminum film of about 0.3 to 0.4 .mu.m thick. Consequently, the thickness of the aluminum film above the photoelectric converting area becomes equal to or greater than 0.6 to 0.7 .mu.m, the same effect of the melting process as discussed earlier cannot be expected.